The "Acid Test" for lead carbonate Minerals and also Carbonate Rocks

Article by: Hobart M. King, PhD, RPG

Hydrochloric acid on limestone: This video clip demonstrates what happens as soon as one fall of dilute (10%) hydrochloric mountain is placed on a item of limestone. Protective glasses and also gloves room recommended when doing the mountain test.

You are watching: What does the acid test tell you about a mineral


What is the acid Test?

To many geologists, the term "acid test" way placing a drop of dilute (5% come 10%) hydrochloric acid on a absent or mineral and also watching for bubbles that carbon dioxide gas to be released. The bubbles signal the visibility of lead carbonate minerals such together calcite, dolomite, or among the minerals noted in Table 1.


The bubbling release of carbon dioxide gas can be therefore weak that you need a hand lens to observe single bubbles slowly cultivation in the autumn of hydrochloric acid - or so vigorous that a flash of effervescence is produced. These variations in effervescence vigor room a an outcome of the kind of lead carbonate minerals present, the quantity of lead carbonate present, the bit size the the carbonate, and the temperature the the acid.


Hydrochloric acid on limestone: This video clip demonstrates what happens when one autumn of dilute (10%) hydrochloric mountain is put on a piece of limestone. Protective glasses and gloves are recommended as soon as doing the acid test.


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Magnesite: The mineral magnesite, which has a chemistry composition that MgCO3, will certainly effervesce weakly with warmth hydrochloric acid and an extremely weakly through cold acid. Specimen is about 6.4 cm across.


What causes the Fizz?

Carbonate minerals are unstable in contact with hydrochloric acid. As soon as acid starts to effervesce (fizz) ~ above a specimen, a reaction comparable to the one shown listed below is taking place.

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On the left next of this reaction, the mineral calcite (CaCO3) is in contact with hydrochloric mountain (HCl). These react to kind carbon dioxide gas (CO2), water (H2O), dissolved calcium (Ca++), and dissolved chlorine (Cl--). The carbon dioxide bubbles the you watch are proof that the reaction is taking place. As soon as that occurs, calcite or another carbonate mineral is present.

Many other carbonate mineral react through hydrochloric acid. Each of these minerals consists of one or an ext metal ions linked with a lead carbonate ion (CO3--). The chemistry of these reactions is comparable to the calcite reaction above. The mineral reacts through hydrochloric mountain to produce carbon dioxide gas, water, a liquified metal ion, and also dissolved chlorine. The reactions for magnesite (MgCO3) and also siderite (FeCO3) are shown below.

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Calcite: This transparent specimen that calcite mirrors cleavage that is characteristics of the mineral. Calcite, v a composition of CaCO3, will certainly react strongly through either cold or warm hydrochloric acid. Specimen measures around 10 cm across.

Acid reactions of carbonate Minerals
MineralChemical CompositionCold acid ReactionWarm mountain Reaction
AragoniteCaCO3strongstrong
AzuriteCu3(CO3)2(OH)2yesstrong
CalciteCaCO3strongstrong
DolomiteCaMg(CO3)2weakyes
MagnesiteMgCO3very weakweak
MalachiteCu2CO3(OH)2yesyes
RhodochrositeMnCO3weakyes
SideriteFeCO3very weakweak
SmithsoniteZnCO3weakyes
StrontianiteSrCO3yesyes
WitheriteBaCO3weakweak
Table 1: A perform of commonly and occasionally encountered lead carbonate minerals with their chemical formula and also reactions to cold and warm hydrochloric acid. Test results can vary since of weathering, vault testing, contamination, and also specimen purity.

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The Vigor of lead carbonate Reactions

Careful monitoring is important since some lead carbonate minerals reaction vigorously and others right react through cold acid.

The carbonate mineral the is most generally encountered by geologists is calcite (CaCO3). Calcite is a "ubiquitous" mineral. Ubiquitous means "found everywhere." Calcite occurs in igneous, metamorphic, and also sedimentary rocks and also is the most typically encountered lead carbonate mineral. If you ar one fall of cold hydrochloric acid on calcite, the entire drop of acid will certainly erupt with bubbles and also a vigorous fizz will certainly last for a few seconds.

Dolomite CaMg(CO3)2 is an additional commonly encountered lead carbonate mineral. If you ar one drop of cold hydrochloric acid on a piece of dolomite, the reaction is weak or no observed. Rather of seeing an evident fizz, friend will see a drop of acid on the surface of the mineral that could have a couple of bubbles the carbon dioxide gas slowly cultivation on the dolomite surface.

However, if warmth acid is put on dolomite, an noticeable fizz will certainly occur. This occurs since the acid and rock react an ext vigorously at higher temperatures.

If you ar a drop of hydrochloric mountain on powdered dolomite, a visible reaction will occur. This is because the surface ar area has been increased, making more dolomite easily accessible to the acid. (You can quickly make dolomite powder by scratching a specimen of dolomite across a streak plate. Climate test the powder by placing a autumn of hydrochloric acid on the powder. One more easy way to develop a little amount that mineral powder is to scratch the specimen v a nail.)

Different lead carbonate minerals have various responses to hydrochloric acid. A perform of common and also occasionally encountered carbonate minerals is offered in Table 1 v their chemistry composition and their relative reaction through cold and also warm hydrochloric acid.

When a mineral has a weak response to acid, you have to be observant and patient to watch it. For example, magnesite has a really weakreaction through cold HCl. If you flour a tiny amount of magnesite ~ above a streak plate and place a drop of acid on it, you could not watch any activity for several seconds. Then, as tiny bubbles begin to kind on particles of magnesite, the fall of mountain will appear to flourish larger in size. The occurs together carbon dioxide is liberated native the mineral and also displaces the water. Observing the formation of bubbles v a hand lens have the right to be helpful.


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Dolostone: Dolostone is a sedimentary rock composed mostly of the mineral dolomite, which has actually a chemistry composition that CaMg(CO3)2. Dolomite will certainly effervesce weakly v cold hydrochloric acid, producing a couple of bubbles. The reaction is more noticeable once the acid is warm and/or the stone is powdered. The specimen in the photograph is about 10 centimeter across.


The acid Test on Rocks

LIMESTONE, DOLOSTONE, and also MARBLE

Some rocks contain carbonate minerals, and also the acid test can be supplied to aid identify them. Limestone is composed nearly entirely of calcite and also will create a vigorous fizz through a drop of hydrochloric acid. Dolostone is a rock written of almost entirely of dolomite. It will develop a very weak fizz when a autumn of cold hydrochloric acid is put upon it, a more obvious fizz once powdered dolostone is tested, and a more powerful fizz when hot hydrochloric acid is used.

Limestone and also dolostone have the right to be a little an ext complex. Castle are occasionally composed that a mixture that calcite and also dolomite and have mountain reactions that space deceptive. A dolostone deserve to contain sufficient calcite to fool you right into calling that a limestone. For these rocks the acid test could not be sufficient for a confident identification - but at the very least you will know that the rock has a significant carbonate mineral content.

Marble is a limestone or a dolostone that has actually been metamorphosed. That will have actually an mountain reaction the is comparable to the limestone or dolostone from which it to be formed.


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Other Applications the the "Acid Test": Geologists have the right to use dilute hydrochloric acid to help identify the cementing agent of sandstones. They ar a drop of dilute HCl ~ above the sandstone and closely observe. If calcite is the cementing agent, one effervescence will certainly occur and some the the sand grains could be liberated. A hand lens or tiny microscope is offered to do the observations. The photo above is a intensified view that a piece of Oriskany Sandstone, one Ordovician-age absent unit from the Appalachian basin that serves together a organic gas reservoir and also a natural gas storage unit. Oriskany sandstone is often cemented by calcite.


OTHER ROCKS the FIZZ

Always remember that "calcite is ubiquitous." (Ubiquitous way that that is found almost everywhere.)

Many rocks contain tiny amounts of calcite or various other carbonate minerals. Every one of these can produce a fizz even though the carbonate is just a minor component of a rock"s composition. These rocks can contain little veins or crystals of lead carbonate minerals that produce a fizz in call with acid. This veins and also crystals deserve to be so tiny the they room not visible to the unaided eye. This tiny amount of carbonate can fizz the first time a autumn of acid is used but it is in depleted and not fizz if acid is used a second time to the same location on the rock.

Some sedimentary rocks room bound together with calcite or dolomite cement. Sandstone, siltstone, and also conglomerate sometimes have calcite cement the will develop a vigorous fizz with cold hydrochloric acid. Part conglomerates and breccias contain clasts of carbonate rocks or minerals that react with acid.

Many shales to be deposited in maritime environments and also contain enough calcium lead carbonate to create a vigorous acid fizz. These shales were created when mud was deposit in one environment similar to or adjacent to wherein limestone to be formed. They space composed of sedimentary clay mineral intermixed through a small amount the calcite. Castle are well-known as "calcareous shales."

Don"t allow an mountain fizz to guide the identification process. In many situations it will instead add detail to your monitoring such as: "calcareous shale" or "sandstone with lead carbonate cement." This is valuable information.


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Vinegar have the right to be supplied for the acid test: Vinegar can be a safe, economical and also easy-to-obtain "acid" because that identifying calcite and also dolomite. Vinegar is dilute acetic mountain that produce a very weak reaction with calcite and also dolomite - finest observed through a hand lens.

The "Vinegar Test"

Vinegar is a dilute acetic acid systems (about 5% come 10%) the produces a weak effervescent reaction v calcite and dolomite. It can be used instead of hydrochloric acid for introducing students come the acid test. Vinegar is simple to obtain, inexpensive, and also safer to use than hydrochloric acid.The effervescence using vinegar usually calls for a hand lens because that clear observation and also is only observable with carbonate mineralsthat have a strong reaction v hydrochloric acid. Vinegar is frequently used once the mountain test is component of a precollege course. Protective gloves, glasses, file towels, and also immediate accessibility to one eyewash station space recommended.

EXTREME mountain REACTIONS

A few rocks can produce an extreme reaction v hydrochloric acid. These space usually rocks composed of calcite or aragonite withabundant pore room or incredibly high surface areas. Some specimens that chalk, coquina, oolite, and tufa space examples. When a fall of dilute hydrochloric mountain is placed on this specimens, an eruption of acid foam deserve to rise up turn off of the rock and also spread come an unanticipated diameter. The reaction is really brief (and might not be repeatable), however it is therefore sudden and vigorous that it have the right to surprise an inexperienced person. This description is for one drop of acid. If much more is supplied an even more vigorous reaction will occur. (These too much reactions will not take place with every specimen of this rocks. Be aware when trial and error them or presenting them come students because that testing.)

The exceptionally vigorous reaction of cold hydrochloric acid through these specimens occurs since the rocks room so porous or due to the fact that they have actually a very high surface ar area under a solitary drop the acid.

TEST UNWEATHERED MATERIAL

Calcite and also other carbonate minerals have actually a short resistance to weathering and also can be attacked by mountain in organic waters and also soils. When experimentation material that has actually been exposed in ~ Earth"s surface, that is an extremely important to test unweathered material. A fresh surface have the right to usually be derived by breaking the rock.

DECEIVED by POROSITY!

Some rocks space porous and also contain a reservoir the air. Small amounts of air escaping into a drop of acid from listed below can provide the figure of a gentle mountain reaction. Don"t it is in fooled. If you ar a autumn of mountain on some sandstones, a couple of bubbles will arise out of pore spaces. It"s no a carbonate cement. To prevent this problem scratch the rock across a streak plate and test the powder or the grains that are produced.


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The best way to learn about minerals is to study with a collection of small specimens that you deserve to handle, examine, and also observe their properties. Inexpensive mineral collections are available in the londonchinatown.org Store.


Contamination in Mineral to know Labs

When student are given minerals to identify, two instances can reason problems v their work.

1) In mineral to know labs, part students are ready to call any mineral that produces an acid reaction "calcite" or an additional carbonate. However, calcite is a ubiquitous mineral and also it is often present as one intimate component of various other mineral specimens and rocks. This can create a false mountain reaction. To stop being misled, college student should always be cautioned to check a specimen"s identity with multiple properties. If a specimen fizzes v acid however has a Mohs hardness of seven and breaks with a conchoidal fracture, climate it definitely isn"t calcite!

Depending top top the experience of the students, specimens that are an extremely true to your properties have the right to be presented to the class, or specimens v some difficulties can be used. Several minerals discovered in the ar will not be certain true to properties. It"s far better to find out that great in the lab and go right into the ar with wisdom.

2) since calcite is one of the index minerals that the Mohs Hardness Scale, that is often used to test the hardness of mineral specimens. This have the right to place small amounts of calcite potentially on every unknown specimen in the lab! Don"t assume that a solitary acid reaction is correct. Check the specimen in a 2nd location if you doubt that contamination has actually occurred.

In a mineral to know lab, barite is typically confused through calcite due to the fact that of contamination. The barite can naturally contain little amounts of calcite, or the hardness experimentation of a previous student could have left little amounts of calcite on a barite specimen.Students are often attracted to an identification together "calcite" simply because of the mountain test. If that mineral exhibits a bit ofcleavage and also is not very hard, then countless students will arrive in ~ an not correct identification.


Acid check Safety

Hydrochloric acid, effectively diluted to a 10% concentration, can cause irritation if it contacts the skin or eyes. It can likewise fade clothing. Hydrochloric acid must kept in clearly-labeled dispensing bottles and also used through quick and also easy access to file towels, water, and an eyewash station. Safety glasses and protective gloves room recommended. If skin call occurs, the area should be flushed with plenty of water. If eye contact occurs, the eye have to be purged for 15 minutes through plenty the water. If a call lens is worn, the eye should be flushed, call lens removed, and flushing continued. Seek prompt clinical attention because that eye contact.Specimens that space tested with acid must be rinsed after testing to eliminate or dilute unreacted acid.

Limiting Frivolous Acid usage in Labs

Most students room intrigued with the mountain test and also want to shot it. To border frivolous acid use, students should be instructed to usage a single drop of mountain for the test and also to only test specimens when carbonate minerals space suspected. If the is no done, some students will usage the acid frivolously. This habits is encouraged if the great is equipped with large acid bottles that space filled come the top. However, if the acid bottles are small and nearly empty at the beginning of class, students usually ration their use of the acid to suitable amounts. Small, virtually empty party makes less acid available to spill.


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Acid dispensing bottles: little acid dispensing bottles work well because that the acid test. Castle dispense the mountain one-drop-at-a-time and will not spill if they room knocked over. If you room a teacher supervising the acid test in a classroom, offer students little bottles that are virtually empty. That will mitigate the quantity of frivolous acid usage that can otherwise occur. Brand the bottles clearly and instruct student in mountain use prior to making them available.


Acid party Selection

The form of bottle selected for dispensing the acid is important. Activities supply stores offer bottles that are designed for dispensing acidone-drop-at-a-time. The lid is constantly on these bottles (except as soon as they are being cleaned or refilled), and they perform not create a pour out whenthey are knocked over. Bottles with a removable lid that has a squeeze pear dispenser will be occasionally knocked over as soon as the lid is offif they room being supplied by normal humans.

Acid dispensing bottles need to be make of strict plastic v a tiny opening which allows acid come be conveniently dispensed one-drop-at-a-time. Soft dispensing bottles or bottles through a bigger opening can dispense a large amount of acid through an inadvertently squeeze.

Sources of Hydrochloric Acid

Hydrochloric mountain diluted come a 10% solution cannot it is in purchased in most communities. The finest place to purchase commercially all set solutions is from a activities supply company. Purchasing the ready-for-use is the recommended method to achieve it. Don"t try to prepare your very own solution if friend don"t know specifically what you space doing and have an fitted laboratory. Your chemistry room might be able to assist you v ordering acid. Part generous chemists will prepare a 10% solution for you.

Mineral Specimens together "Consumables"

Mineral specimens the are supplied properly in the science classroom or activities will need to be changed frequently.Students will certainly be investigating them with hardness tests, streak tests, mountain tests and other experiments. All of thesetests damages the specimen and make it much less fit because that the next team of students. To store the mountain test indigenous fouling your whole collection, ask students to rinse specimens after testing with acid and also limit testing to only when it is needed.


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